Molecular Cell Biology
Hemoglobin: Normal Synthesis and Mechanism of Action and Abnormality in Sickle-cell Anemia
Hemoglobin is the oxygen-carrying metalloprotein, found in erythrocytes, that transports oxygen from the lungs to rest of the body. While oxygen is used to metabolize nutrients, hemoglobin takes up the excess waste carbon dioxide produced by the cells and facilitates its expulsion from the body. Hemoglobin is coded for by the gene HBB with its locus at 11p15.5 (Smith et. al., 1999), and assumes a globular conformation, constituted of 576 amino acids in four separate polypeptide monomers. Each hemoglobin molecule has a globin part and a heme part. There are two alpha-globin subunits composed of 141 amino acids each and two beta-globin polypeptides, each 146 amino acids long (Lukin et. al., 2004). Each globin polypeptide houses a heme subunit so the hemoglobin molecule has 4 globins and 4 heme molecules. Each of the globins is folded into a secondary and tertiary structure.
During the synthesis of fetal hemoglobin, instead of the beta-globins, there are gamma-globins, which have a higher affinity for oxygen than adult hemoglobin, which helps in the transfer of oxygen from the mother to the fetus. In adults, these are replaced with beta-globins. The alpha globins are coded for by alpha1 and alpha2 genes located on chromosome 16. There are promoter elements 5’ to each gene, while the Locus Control Region (LCR), a powerful enhancer region, is required for the expression of alpha1 and alpha 2 (Kim et al., 1992). This is located several kilobases upstream of the alpha globin locus. Each globin, upon synthesis, is folded into a structure similar to that of myoglobin. This fold is an arrangement of the helices which form a pocket that encloses and binds the heme prosthetic group (Lukin et. al., 2004).
Each heme group consists of an organic component called porphyrin and a central iron atom, which binds to the NE2 atom of the F8His residue of the polypeptide chain. This is where the binding of the heme iron takes place. The heme iron binds oxygen as a ligand at the distal side of the heme plane, opposite to the proximal His. Oxygen binding to one heme group induces substantial structural changes at the heme sites of the hemoglobin, which increase the cooperativity and facilitate the further binding of oxygen to the heme group. This binding is tightly regulated by pH and carbon dioxide concentration in blood, where the oxygen affinity of hemoglobin goes down as the pH decreases from 7.4. In other words, hemoglobin is more likely to bind oxygen in alkaline conditions, which are an indicator of elevated levels of carbon dioxide. This variability in the affinity for oxygen is directly related to the amino acid sequence that forms hemoglobin. Consider the following sequence of the amino acids in the N-terminus of hemoglobin:
The interaction of the side chain of histidine with a negatively charged glutamate in the same chain forms a salt bridge at acidic pH where the histidine is protonated. At high pH the side chain of histidine is protonated and lacks the ability to form the salt bridge. This makes it more available to bind oxygen. Slight changes in this sequence can result in major problems with the function of hemoglobin. This is observed in the disease Sickle cell anemia.
Sickle cell anemia is an autosomal recessive genetic disorder and is caused by a variant of the HBB gene; Hb S. the Hb S variant causes the amino acid sequence in the N-terminus of hemoglobin to change to:
As the hydrophobic amino acid valine takes the place of hydrophilic glutamic acid at the sixth amino acid position of the HBB polypeptide chain. This substitution creates a hydrophobic spot on the outside of the protein structure that is attracted to the hydrophobic region of an adjacent hemoglobin molecule's beta chain. This polymerization of Hb S molecules into rigid fibers causes the "sickling" of red blood cells.
This polymerization can only take place in the deoxygenated state of hemoglobin. When hemoglobin binds with oxygen, the molecule depolymerizes. This cycling between polymerization and depolymerization causes red blood cell membranes to become rigid, which makes them more susceptible to breakage. Due to their fragile condition, blockage of erythrocytes can cause blockage of capillaries, which can cause pain and can damage organs.
Smith, Z. E., Higgs, D. R. 1999. The pattern of replication at a human telomeric region (16p13.3): its relationship to chromosome structure and gene expression. Human Mol. Genet. 8(8): 1373-1386.